Abstract
A new orthogonal-diagonal steel open-web sandwich floor system has been developed by the authors in the past few years to replace the conventional methodology. This floor system owns excellent structural mechanical characteristics to be applied in civil engineering applications. The three-dimensional structural characteristics are achieved by letting the top and bottom chords of the space truss that constitutes the floor inclined 45° toward the edge of the boundary lines. The present work focuses on the fabrication and erection of the proposed floor and its structural behavior subjected to partial loadings. Field partially-loaded test was conducted and the performance was evaluated in terms of the deflection distribution and stress response at different load levels. A three-dimensional numerical finite element model was developed and calibrated against the test results. The results demonstrate that the new floor system offers satisfactory structural performance.
Introduction
When floors are being designed for commercial or residential buildings, there has to be consideration on the side of the designers to evaluate the effect that the design would have on the whole structures. The chosen designed depth of floor might have profound impact on the entire height of the buildings. Consequently, a desire to minimize the depth of floor in buildings, based on innovative floor assemblages, is practical for both engineers and architects.
The concept of the floor-to-floor height is the distance from the top of a floor to the top of the next floor. This height is made up of three components: the distance from the top of the floor to the top of the architectural ceiling; the “sandwich” which contains parts of the communication, fire protection, electrical, and plumbing systems; and the depth of the floor. Designers should attempt to separately minimize the depth of one of the three components or allow them to share functions in the same space.
Steel-deck composite floor is increasingly used in high-rise buildings (De Silva and Thambiratnam, 2009). Some study on the behavior of steel-deck composite floors has been conducted by Da Silva et al. (2003), El-Dardiry and Ji (2005) and Behnia et al. (2013). The finite element (FE) methods were used to investigate the structural behavior of composite floors subjected to human-induced activities. Stub-girder system is one kind of systems that architects and engineers set out to minimize floor-to-floor heights. The idea behind the development was proposed by Colaco (1972) to address the problems that conventional floor systems had in accommodating mechanical ducts. Chien and Ritchie (1993) made some changes to the original proposed stub-girder system, one of which is the truncation of girder bottom chord to accommodate services near supports. This change provides several merits over original one in terms of crack control near the column connection, availability of more room for stud connection, and wider duct space. Girder-slab is a recent try to use a prestressed, precast concrete plank and steel system to take the place of traditional plank and bearing wall construction (Naccaroto, 2000). Initial testing of the system demonstrated that composite action was developed between the precast hollow-core slab, the integral steel girder, and the grout. This system is efficient in reducing the construction schedule to 75% and meanwhile, having the ability of maintaining equivalent floor-to-floor heights compared to cast-in-place concrete construction. Itzler (2004) proposed a new concept to combine autoclaved acrated concrete (AAC) floor panels with structural steel to realize composite action. Research showed that the reduced dead loads in this system could lead to a drop in both the foundation size and the seismic loading on the lateral load resisting system. Hassett (2004) describes a floor system made up of closed ribs welded on the underside of a thin plate. Electrical and plumbing conduit runs across the space between the ribs. In addition, gypcrete is arranged on top of the plate to offer a finished surface as well as sound and vibration dampening and fire rating.
Another building system that can minimize the floor-to-floor heights is the staggered-truss. The main idea involving a double-planner system of steel framing was proposed by a technical group from the Department of Architecture and Civil Engineering at M.I.T. This system is made up of external columns supporting story-deep trusses spanning the full transverse width of the building on adjacent column lines (Cohen, 1986). In some cases, the height of the building may be limited by the shear capacity of the floor. However, the depth of floor can be minimized as the floor spans may be short bay lengths, providing two column bay spacings for room arrangements.
A typical open-web steel joist floor system uses concrete slabs, supported by ribbed steel decks bearing on the joists. The main components of the systems include the concrete slab, the steel deck, and the joists. The top chord of the joist and the overlying concrete slab would compositely act once the concrete has cured. The joists are composed of cold-formed or hot-rolled steel. Welded shear studs or specially designed top chord of joist should be provided to ensure the shear transfer, which allows the concrete slab to act as compress flanges.
Space trusses can also serve as the floor system. They are made of highly indeterminate three-dimensional lattice networks. The spanning of long distances in two directions was achieved depending on disciplined member repetition and geometric modularity. Consequently, this type of system offers the architects and engineers an opportunity to pick up a system that can span in two directions. A new space truss system named Catrus Space Truss was developed by a group of researchers in the University of Dundee (El-Sheikh, 2000) in order to achieve the objective of low cost, sound structural behavior, and constructional advantages. The main feature of this new system is its simple jointing system. The top and bottom chord members are continuous across the joints. Single bolts are used to directly connect the members. The chord and diagonal members are stacked above each other and therefore, producing joint eccentricity and chord member continuity.
The structural solutions that attempt to decrease the floor-to-floor heights were reviewed in this section. While each system offers a unique set of merits, their one-way spanning behavior, when applied to the structures with the length-to-width ratio greater than 1.5, makes them vulnerable to deflection and stress concentration problems. From practical point of view, the authors believe that an additional alternative should be investigated to provide two-way spanning performance. This innovative alternative is an orthogonal-diagonal steel open-web sandwich floor system. As full-scale testing provides an important means for acquiring in situ information of a constructed facility (Au et al., 2012; Papadimitriou et al., 2005; Yue and Katafygiotis, 2003), this article presents the field fabrication and erection of the proposed innovative floor system. The field test is described in detail with particular reference to its deflection and stress distribution under partial loadings. The article also contributes to the establishment of the FE model to validate the excellent three-dimensional characteristics of the floor.
Proposed system fabrication and erection
Introduction
For industrial and residential buildings with a length-to-width ratio in plan greater than 1.5, the performance of the floor is mostly close to the one-way spanning slab. The load from the floor would transfer to the short-span side to a large extent. The three-dimensional characteristics of the space floor cannot be fully recognized. Moreover, the deflection of the floor would significantly increase and the stress concentration may occur. This in return would enlarge the cross-section of the selected beams and result in an increase of the steel assumption. To solve the issues mentioned above, an innovative orthogonal-diagonal steel open-web sandwich floor system is proposed by the authors. Since this new floor system was developed in 2012 and awarded patent by the State Intellectual Property Office of the P.R. China, it has been applied to several real civil engineering projects, such as the Guizhou Natural Factory (Figure 1(a)) and the Multi-Story Building of Hunan Xiangtan Industrial Park (Figure 1(b)).
Projects in progress with the proposed floor system: (a) Guizhou Natural Factory and (b) Multi-Story Building of Hunan Xiangtan Industrial Park.
Figure 2 shows the force transfer in the square floor. When the spans along two orthogonal direction equal, that is, lx = ly, and concentrated force P is applied at the center of the floor, the forces transferred to the two orthogonal directions are the same, that is, Px = Py = 0.5P. Consequently, in this case, the force transfer is smooth if the floor is along the edge of the building.
Force transfer in square floor:(a) square floor and (b) force transfer in two directions.
Many industrial buildings however, own the rectangular plan as shown in Figure 3. Assuming the orthogonal-orthographic floor system is applied and the length (lx) to width (ly) ratio of the floor n = lx/ly as shown in Figure 2, when the concentrated force P is applied to the center of the floor, the forces transferred to the long span lx and short span ly are Px = P/(1 + n3) and Py = Pn3/(1 + n3), respectively.
Force transfer in rectangular floor (a) Rectangular floor and (b) Force transfer in two directions.
In the studied project in this article, the length-to-width ratio of the floor n = 143.5/21 = 6.83. The loads transferred to the short span and long span of the floor are Py = 0.997P and Px = 0.003P, respectively. Thus, the two-way spanning performance is not quite significant. If the orthogonal-diagonal floor system is utilized and the floor (or the so-called open-web beam) is at 45° to the edge of the building as shown in Figure 2(a), the forces transferred to the open-web beams along two directions would be Px = Py = 0.5P. Therefore, the two-way spanning performance is fully realized.
Fabrication
Orthogonal-diagonal steel open-web sandwich floor system, as its name tells, has the two ways “beams” orthogonal to each other and as well, inclined at 45° to the concrete slab edge. In general, it is a new type of space truss systems. This system consists of steel structural tees forming the top and bottom chords and square tubes forming the vertical web members as illustrated in Figure 4. Concrete is poured on the upside of the top chords to constitute the slab. Shear studs are also provided on the surface of the top chords to ensure the composite action between the steel and the concrete slab. Q235 or Q345 is generally used for the steel, where Q represents the yield strength and the number behind it denotes the value for the yield strength. Namely, Q235 is the steel grade with the yield strength of 235 MPa. C20 or C30 concrete is the typical selection for the concrete slab with a compressive strength between 13.4 and 20.1 MPa. The thickness of the slab is normally less than 80 mm due to the small size of grids which were formed by the top or bottom chords. The diameter of shear studs usually ranges from 8 to 16 mm.
Configuration of the proposed floor.
Erection
Figure 5 was taken as an example to introduce the basic procedures to erect the orthogonal-diagonal steel open-web sandwich floor system. The general panelized erection sequence begins with the division of the floor panel into several specified units A, B, and C (detailed in Figure 6) as shown in Figure 5. The next step is to fabricate these unique units by welding individual members together and deliver them to the job site. After the framing wall on the first story is constructed, the fabrication units A, B, and C are arranged in the right position by crane. Since the floor system is composed from the steel open-web beam and the concrete slab, the concrete slab would largely increase the stiffness of the floor. Therefore, in order to provide the system with sufficient stiffness, the temporary propping is used before the concrete is poured on the steel open-web beam (Figure 7) and is removed after the concrete has reached its design strength. The assembly of different units is achieved by connecting the webs of the chords using high-strength bolts and connecting the flanges of the chords using complete joint penetration (CJP) welds. The next step is to fix the shear studs on the flanges of the top chords and the concrete is poured to form the floor slab. The steel-concrete composite open-web sandwich floor could then act as an integral after the concrete has cured. The same erection process is used to construct the other stories of floors.
Division of a typical floor by units A, B, and C.
Details of units A, B, and C: (a) unit A, (b) unit B, (c) unit C, (d) section 1-1 for units A and B, and (e) section 2-2 for unit C.
Scaffolds supported floor.
Design method and philosophy
FE methods cannot always do that on a general basis suitable for a practicing engineer. To utilize traditional theory in order to find solutions for the system, simplifications and assumptions should be made to reduce the model’s complexity to a reasonable level. The design philosophy is based on the following approximations:
Plane sections remain plane (compatibility) after the deformation of the open-web beam.
The number of grid along the short span is not less than 5 in order to guarantee the entire performance of the floor.
The calculated moment is increased by 20% for the design of top and bottom chords and web members to consider the impact of the local moment.
The contribution of the concrete slab to the strength is neglected and is considered as safety reserve.
When calculating the deflection of the steel open-web sandwich floor system, the calculated deflection is divided by 0.65 to obtain the maximum deflection in order to consider the negative effect of the shear deformation of the open-web beam.
Based on the principle of equivalent bending stiffness, the open-web beams are converted to conventional beams with the same depth, and thickness of web and flange by altering the width of the flange. The stress and displacement of the floor can then be obtained by the application of the general commercial software such as Midas and PKPM. The actual axial force and shear force in the top and bottom chords can be derived from equations (1) and (2), respectively
where N is the designed axial force in the top and bottom chords, M is the calculated bending moment in the conventional beam (multiplied by a factor of 1.2 to consider the effect of local moment), h is the depth of the floor,
The shear force in the web member is the difference in axial force between the two adjacent chords.
Field partially-loaded test
Investigated structural model
The investigated structural model was based on a real orthogonal-diagonal steel open-web sandwich floor, the abutment span of which covers 63 m by 42 m with a total area of 2646 m2. The structural system consisted of a typical orthogonal-diagonal steel open-web sandwich floor of an industrial building. The floor studied in this work is supported by grid framing wall.
The floor height is 920 mm including two parts: the depth of the steel beam (840 mm) and depth of the concrete slab (80 mm). The steel beams and the concrete slab are connected via welded shear studs to act compositely. The grid size is 2.475 m×2.475 m. The top and bottom chords are fabricated by steel structural tee sections with a section of T200×150×8×13. The web members are square tubes with a section of □200×200×8×8. Stiffened plates are added to four sides of the square tubes to improve the shear transfer capacity. The concrete slab has a 20.1 MPa (Grade C30) specified compressive strength. Figure 8 depicts the geometric characteristics of the abutment span of the floor.
Floor dimension and the loading area.
Material properties
Three tensile coupons cut from the sheets (used to make the square tube) and the flange and the web of the steel beams were tested to determine the modulus of elasticity (
Coupon details.
Material properties of steel.
Material properties of concrete.
Loading procedure
In this investigation, the partially distribution loadings were applied to the structural model. The live load considered in this analysis (9.80 kN/m2) corresponds to 163.30% working load. The load distribution was considered symmetrically centered along axis 16 on the slab panels, as shown in Figure 8. Cement block with a dimension of 240 mm (length)×115 mm (width)×53 mm (depth) was used to simulate the vertical distribution load. Each block weights 3.50 kg and 35 blocks approximately occupy 1 m2. The equivalent distribution load for one layer of blocks is 1.23 kN/m2. Note that there would be arch action if the deflection is quite significant and the connection among the brick is tight. As we can see from Figure 15, the maximum deflection in the test in 28.26 mm, which is quite small and the floor is still in the elastic range. Considering the small deflection and lack of constraint among bricks, the arch action could be ignored in the analysis.
The floor was pre-loaded to 3.68 kN/m2 through three load stages at intervals of 1.23 kN/m2 and then unloaded. The formal loading was controlled by the load increase. The floor was loaded to a level of 1.23, 2.45, 3.68, 4.90, 6.13, 7.35, 8.58, and 9.80 kN/m2, respectively. The entire test lasted for 2 days. The data were collected automatically by a computer. The on-site photo of loading is shown in Figure 10. The unload process was completed in two steps, the first of which was set to 4.90 kN/m2 and the second was set to 0.
Loading in progress.
Instrumentation
In total, 21 displacement transducers (DTs) and 350 strain gauges were arranged to monitor the structural movement, crack development, and the stress distribution during the test. Due to the symmetry of the structural arrangement and the loading area, most of the measured points are concentrated in one quarter of the plane. In addition, several other locations were selected as control points. The arrangement of DTs is specified in Figure 11. Figure 12(a) shows the location where the strain gauges were arranged. Each location corresponds to two strain gauges as illustrated in Figure 12(b). For example, A40 and B40 represent the strain gauge in the top chord and bottom chord, respectively, at the location 40. Fifty strain gauges were used to measure the stress distribution in five web members as shown in Figure 13. The web members, namely J1, J2, J3, J4, and J5, have their four faces labeled as 1, 2, 3, and 4. The strain gauges located at the top, center, and bottom of each side were marked by the letter T, C, and B, respectively. J1-1(TCB) denotes that face 1 of web member J1 has three strain gauges arranged at the top, center, and bottom, respectively. J1-3(TB) represents that two strain gauges are located at the top and bottom, respectively, of face 3 of web member 1. Meanwhile, 14 strain gauges were attached on the surface of the concrete slab to monitor the crack development as detailed in Figure 14.
Arrangement of DTs.
Layout of strain gauges in the chords: (a) arrangement of strain gauges and (b) cross-section view.
Layout of strain gauges in the web members.
Layout of strain gauges on the concrete slab.
Test results and discussion
General observation
The test proceeded in a smooth and controlled fashion. There was no physical failure in the concrete slab or steel members during the test. It was observed that the floor essentially behaved in an elastic manner. The measured strains were less than the yielding strain, and the displacements exhibited linear characteristics. No fracture was observed in the floor before the load reached 6.125 kN/m2. As the load arrived at 7.35 kN/m2, small fractures occurred on the surface of the concrete slab within the loading area. The width of the fracture was less than 0.1 mm. After careful examination, it demonstrated that these fractures existed only on the surface and did not progress into the slab. As the load reached 9.8 kN/m2, the width of the fractures was still less than 0.1 mm and remained on the surface while extended to an area equal to three times loading area.
Deflection
Figure 15 illustrates the deflection of part of the floor when the load reached 6.125 kN/m2. It can be seen that the flexural deformation dominates and the deformed floor exhibits oval shape, where the maximum deflection occurs at the center and deflection decreases gradually to 0 toward the edge. The maximum deflection at point 4 is 28.26 mm, which equals 1/743 of the span. This meets the requirement specified in Chinese Code for Design of Steel Structures GB 50017-2003.
Deflection of the floor.
Stress distribution
The measured strains were converted to the corresponding stresses by the stress–strain relationship to investigate the stress distribution and development in the chords of the orthogonal-diagonal steel open-web sandwich floor. It should be noted that the measured cracks at the concrete surface using strain gauges were within the limits and satisfied the serviceability limit states. Figure 16 shows the response of the stress in the top and bottom chords at the locations of L3 and L4 (specified in Figure 12(a)) and the corresponding predicted results. The calculated data were based on the FE model established in section “FE simulation.” It can be observed that all measured points had almost identical stress development tendency. The stress gradually increased with the incremental load level and slowly decreased as the unloading process progressed. However, the stress in the bottom chord was obviously greater than that in the top chord. It can be explained by the fact that the moment was partly taken by the concrete slab, and thus leads to the reduction in the axial force in the top chord. Moreover, it can be concluded that there was residual stress in the chord after the unloading process, which means that residual deformation remained in the floor.
Stress development of the measured points: (a) top chord of L3 (point A40), (b) bottom chord of L3 (point B40), (c) top chord of L4 (pint A46) and (d) bottom chord of L4 (point B46).
The distribution of the stress in the top and bottom chords along the direction of the span is illustrated in Figure 17. Figure 17(a) shows that the stress in the top chord was changed from tension force to compressive force when the measured location was moved from the support to the center span. The stress reached the maximum value at the support and the center span while reduced to the lowest at the cross-section No. 30, which means that the point of deflection was around cross-section No. 30. It was determined from Figure 17(b) that the stress response in the bottom chord shows the opposite result, where the tension stress at the support was altered to the compressive stress at the center span. Moreover, the value in the bottom chord was significantly greater than that at the corresponding location in the top chord. The entire moment distribution can therefore be estimated: the sagging moment got its maximum value at the center span where the top and bottom chords were subjected to tensile force and compressive force, respectively. The hogging moment developed at the support where the bottom chord was in compression and the top chord was in tension. The deflection point was close to cross-section No. 30, where the stresses in both top and bottom chords were relatively small. Meanwhile, it can be concluded that the impact of local moment on the entire structural behavior was minimal.
Strain distribution along the direction of the span: (a) strain gauges No. A9, A17, A30, A38, A45, and A51 in the top chord and (b) strain gauges No. B9, B17, B30, B38, B45, and B51 in the bottom chord.
Figure 18(a) and (b) shows the response of the stresses in the top and bottom chords along the longitudinal direction. It can be seen that the top chord was subjected to compressive force while the bottom chord was under tension force. The stress gradually decreased with the increase in the distance between the measured point and the center of the loading area.
Strain response along the longitudinal direction: (a) strain gauges No. A51–-A55 and (b) strain gauges No. B51–B54.
Figures 19(a) and (b) and 20(a) and (b) illustrate the stress distribution of the web members. It shows that the web members exhibited horizontal dislocation due to the axial force in the top and bottom chords. Moreover, the stress of the web member J1 at the support was significantly greater than that of J5 at the center.
Strain distribution in web member J1: (a) strain gauges at the top and (b) strain gauges at the bottom.
Strain distribution in web member J5:(a) strain gauges at the top and (b) strain gauges at the bottom.
FE simulation
A three-dimensional FE simulation has been conducted to investigate the structural behavior of the orthogonal-diagonal steel open-web sandwich floor. The proposed computational model adopted the usual mesh refinement methods present in FE method simulations implemented in the ANSYS software. A three-dimensional beam element BEAM188 with tension, compression, torsion, and bending capabilities was used to simulate the columns, the beams, the top and bottom chords, and the web members. The concrete slab was simulated using shell element SHELL63 with both bending and membrane capabilities. Both in-plane and normal loads are permitted. The element has 6 degrees of freedom at each node: translations in the nodal x, y, and z direction and rotations about the nodal x-, y-, and z-axes. The impact of the high-strength bolts was not considered in the model due to the fact that the strength of connections is designed to be not less than the strength of the connected members. No crack occurs in the concrete slab under the designed loadings, and thus, an elastic behavior was considered for both materials (steel and concrete) in the present investigation.
The structural behavior of the connections between the columns and the beams present in the investigated orthogonal-diagonal steel open-web sandwich floor system was assumed to be rigid. When the complete interaction between the concrete slab and steel beams was considered in the analysis, the floor FE model coupled all the nodes between the beams and the boundary of the slab to prevent the occurrence of any slip. Since distance exists between the mid layer of the shell elements and the axis of the beam elements, the shell elements were offset to finalize the intention.
There are four spans in the real project. In order to save the cost of calculation time, two spans were established in the model and symmetric boundary conditions were created. An illustrative FE model for a representative span is illustrated in Figure 21. The steel was made with a steel grade of Q345, Young’s modulus of 2.1×105 MPa, and a density of 7850 kg/m3. The concrete slab has a 3.0×104 MPa modulus of elasticity and a density of 2500 kg/m3. The structure was loaded by its self-weight and distribution dead load from floor (1.5 kN/m2). The impact of wind load and earthquake on the performance of the floor was ignored. The FE simulation is inelastic. The structure remain elastic before the load reaches 13.80 kN/m2, while the ultimate stress of 381.80 kN/m2 was obtained at the loading level of 16.20 kN/m2.
Finite element model.
Structural characteristics
Figure 22 shows the distribution of the moment, axial force, and shear force of the top and bottom chords under the designed loadings. It can be seen from Figure 22(a) and (b) that the local moment in the top chord was greater than that in the bottom chord due to the interaction between the top chord and the concrete slab. The influenced area extended to a length of three spans away from the loading zone and the moment value gradually decreased toward the edge of the floor plate. Axial force distribution given in Figure 22(c) and (d) indicates that the compressive force dominated in the top chord and the tensile force mainly existed in the bottom chord. Moreover, the force in the top chord was almost uniformly distributed while that in the bottom chord changed from the tensile force in the center span to the compressive force at the support. Figure 22(e) and (f) describes the response of the shear force. The shear forces in both top and bottom chords are quite small. This can be explained by the composite action of the floor. In addition, the loads from the floor was transferred directly to the top chord, which results in a greater value observed in the top chord than in the bottom chord.
Force distribution in the chords (kN): (a) moment in the top chord, (b) moment in the bottom chord, (c) axial force in the top chord, (d) axial force in the bottom chord, (e) shear force in the top chord, and (f) shear force in the bottom chord.
The force transfer mechanism can be seen from Figure 22 that the load is transferred to the orthogonal-diagonal beam through the concrete slab and then to the surrounded columns. The force transfer path is simple and the load is able to be transferred to a larger area, which leads to the low stress in the structural members.
Figures 23 and 24 show the stress response in the top and bottom chords along the spans L1–L5 as specified in Figure 12(a). In general, the axial force in the top chord is significantly less than that in the bottom chord due to the composite action between the concrete slab and the steel top chord. The axial force between two nodes in the top cord exhibits a linear behavior because of the presence of the local moment on the top chord. The axial force in the bottom chord has high value in the center span and at the support and smoothly changes from the tensile force to the compressive force when the location moves from the center span to the support. The moment in the chords shows a linear manner and is relatively small comparing with the value of the axial force.
Axial force along the spans L1–L5 (kN).
Moment along the spans L1–L5 (kN m).
Figures 25 and 26 illustrate the moment and the shear force in the web members along the spans L1–L5. While both the moment and the shear force gradually increase from the center span toward the support, the shear force shows a greater increment speed. Stiffeners are added to the four sides of the web member to achieve a length-to-width ratio of less than 1 and thus, improve the moment-resisting and shear-resisting capacity.
Moment in the web members.
Shear force in the web members (kN).
The failure mode of the system is caused by the combined bending and axial load. Take the beam with the maximum stress (L1–L5) as an example, the stress distribution under the load of 13.80 kN/m2 is illustrated in Figure 27. It shows that the stresses in both top and bottom chords are low in the middle. However, the stress reaches a high value of around 343.70 N/mm2 near the support. At this loading stage, the steel near the support starts to yield. When the loading arrives at 16.20 kN/m2, the stress distribution in beam L1–L5 (Figure 28) shows that the stress at the conjunction between the top and bottom chords and the shear members reaches 381.80 N/mm2, which indicates the steel is at the strengthen stage.
Stress distribution in L1–L5 under 13.80 kN/m2.
Stress distribution in L1–L5 under 16.20 kN/m2.
Deflection analysis
The comparison between the measured data and the predicted results at the load level of 6.125 and 9.8 kN/m2 is given in Table 3. In general, the calculated results well corresponds to the experimental ones and most of the deviations have been controlled within 10%. The differences for point 4 (the point with the largest deflection) under 6.125 and 9.8 kN/m2 loads are 11% and 9%, respectively. The deflection distribution of the orthogonal-diagonal steel open-web sandwich floor under the load of 6.125 and 9.8 kN/m2 is given in Figure 29(a) and (b), respectively. The deflected area is like an oval where the largest deflection occurs at the center and the deformation slowly decreases toward the plate edge. The deformed area is around 21 m×49 m = 1029 m2. Figure 30 provides the conventional floor of the steel framed structures under the same loading cases, where the deflected area is approximately 16 m×21 m = 336 m2 and equals only one-third of that of the orthogonal-diagonal steel open-web sandwich floor. This indicates that the orthogonal-diagonal steel open-web sandwich floor system offers good three-dimensional structural characteristics. Both the measured and the predicted deflection developments of point 4 at the center span are depicted in Figure 31. It can be observed that both curves are almost linear, which demonstrates that the floor is still in the elastic range.
Comparison between the measured and predicted results.
Deflection distribution of the orthogonal-diagonal steel open-web sandwich floor (m): (a) 6.125 kN/m2 and (b) 9.8 kN/m2.
Deflection distribution of the conventional floor of the steel framed structures (m): (a) 6.125 kN/m2 and (b) 9.8 kN/m2.
Deflection of point 4.
Comparison with orthogonal spatial steel open-web sandwich floor system
A comparison was carries out between the current 45° and the 90° orientation using the FE model in order to better understand the superiority of the current study. The stress and deflection distribution in the orthogonal spatial floor system are plotted in Figures 32 and 33. It can be observed that there is significant increase in the bending moment, axial force, shear force of the top and bottom chords and the web members for the floor system with the 90° orientation. The forces transferred along the Y-axis direction are obviously larger than those along the X-axis direction, which indicates that the loads are transferred through the short span. A detailed comparison shows that the maximum axial force along the X-axis direction is 280 kN, while that along the Y-axis direction is 341 kN (21.8% higher). Meanwhile, the deflections under the loads of 6.13 and 9.80 kN/m2 for the orthogonal spatial floor system largely increased by 80.6% and 150.5% when compared with the orthogonal-diagonal floor system. Therefore, the orthogonal-diagonal floor system exhibits more reasonable force transfer mechanism and better stiffness.
Stress distribution in the orthogonal spatial floor system (unit for force (kN) and unit for moment (kN m)): (a) moment in the top chord, (b) moment in the bottom chord, (c) axial force in the top chord, (d) axial force in the bottom chord, (e) shear force in the top chord, (f) shear force in the bottom chord, (g) axial force in the Vierendeel girde along Y direction, (h) axial force in the Vierendeel girde along X direction, (i) moment in the Vierendeel girde along Y direction, (j) moment in the Vierendeel girde along X direction, (k) moment in the web members along Y direction, (l) moment in the web members along X direction, (m) shear force in the web members along Y direction and (n) shear force in the web members along X direction.
Deflection distribution in the orthogonal spatial floor system (m): (a) under 6.13 kN/m2 and (b) under 9.8 kN/m2.
Conclusion
This article presented the development of an innovative orthogonal-diagonal steel open-web sandwich floor system. Field test was conducted on an industrial building subjected to partial loading aiming for the evaluation and assessment of the structural performance. Three-dimensional FE model was established to investigate the structural behavior of the proposed floor in terms of deflection and stress distribution. The following conclusions can be drawn from the results presented in this study:
For industrial and residential buildings with a length-to-width ratio in plan greater than 1.5, the orthogonal-diagonal steel open-web sandwich floor system offers better two-way spanning performance than conventional floor and exhibits excellent three-dimensional structural behavior.
The design philosophy of the proposed floor is based on five assumptions. In order to facilitate the calculation, the beams in the system are converted to conventional beams with the same depth, and thickness of web and flange by altering the width of the flange.
The force in the system is transferred to the orthogonal-diagonal beam through the concrete slab and then to the surrounded columns. The failure mode of the system is caused by the combined bending and axial load.
The load–deflection curve shows a linear manner, which indicates that the floor was still in the elastic stage. The maximum deflections under the load of 6.125 and 9,8 kN/m2 were 28.26 and 46.58 mm, respectively. The deformed shape of the floor was like an oval, where the largest deflection occurred at point 4 while the deflection gradually decreased from the loading center toward the floor edge.
There were local moment in the top and bottom chords and the web members. Compressive axial force and tensile axial force dominated in the top chords and the bottom chords, respectively.
Due to the composite action in the floor, the axial force in the top chords was smaller than that in the bottom chords. The axial force between two nodes in the top cords showed a linear distribution. High axial force existed in the bottom chords both at the center span and at the support. The force gradually changes from the tensile force at the center span to the compressive force at the support.
Web members were mostly subjected to moment and shear force. Both the moment and the shear force gradually increased from the center span toward the support, and the value of shear force, however, was greater than that of moment.
The comparison between the current 45° and the 90° orientation shows that the studied 45° orientation could offer better two-spanning effect and greater stiffness, which results in better structural performance.
Footnotes
Declaration of Conflicting Interests
The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.
Funding
The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work is sponsored by A Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD).